Fix WAL replay of truncate operations to cope with the possibility that the

truncated relation was deleted later in the WAL sequence.  Since replay
normally auto-creates a relation upon its first reference by a WAL log entry,
failure is seen only if the truncate entry happens to be the first reference
after the checkpoint we're restarting from; which is a pretty unusual case but
of course not impossible.  Fix by making truncate entries auto-create like
the other ones do.  Per report and test case from Dharmendra Goyal.